The multi-enzyme complex, tryptophan synthase from salmonella typhimurium, has been further analyzed by X-ray diffraction. New very high resolution data have been measured for a mutant of the beta subunit, in the external aldimine form with the amino acids serine and tryptophan, and these structures have been refined. This analysis provides new information about the disposition of residues in the beta active site and their interaction with the intermediates of the beta reaction. Inhibitor complexes of rhizopus pepsin have been analyzed by X-ray diffraction with a view to obtaining more information concerning the mechanism of action. One of these provides a model for the tetrahedral intermediate of the transition state. Examination of the contact distances of this complex provides strong support for the previously proposed mechanism of action. A new method has been presented for the joint refinement of X-ray and NMR data. This method can demonstrate that only very small differences between the structures result from the procedures used, and can highlight real conformational differences caused by physical differences such as crystal packing.